LAS SPECIFICATION VERSION 1.4 – R11 - asprs

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Table 21: Point Data Record Format 10 Item Format Size Required X long 4 bytes * Y long 4 bytes * Z long 4 bytes * Intensity unsigned short 2 bytes Return Number 4 bits (bit 0 – 3) 4 bits * Number of Returns (given pulse) 4 bits (bit 4 – 7) 4 bits * Classification Flags 4 bits (bits 0 – 3) 4 bits Scanner Channel 2 bits (bits 4-5) 2 bits * Scan Direction Flag 1 bit (bit 6) 1 bit * Edge of Flight Line 1 bit (bit 7) 1 bit * Classification unsigned char 1 byte * User Data unsigned char 1 byte Scan Angle short 2 bytes * Point Source ID unsigned short 2 bytes * GPS Time double 8 bytes * Red unsigned short 2 bytes * Green unsigned short 2 bytes * Blue unsigned short 2 bytes * NIR unsigned short 2 bytes * Wave Packet Descriptor Index unsigned char 1 byte * Byte offset to waveform data unsigned long long 8 bytes * Waveform packet size in bytes unsigned long 4 bytes * Return Point Waveform Location float 4 bytes * X(t) float 4 bytes * Y(t) float 4 bytes * Z(t) float 4 bytes * Point Data Record Format 10 is the same as Point Data Record Format 9 with the addition of RGB and NIR values. EXTENDED VARIABLE LENGTH RECORDS (EVLRs) The Point Record Data can be followed by any number of EVLRs. The EVLR is, in spirit, identical to a VLR but can carry a larger payload as the “Record Length After Header” field is 8 bytes instead of 2 bytes. The number of EVLRs is specified in the “Number of Extended Variable Length Records” field in the Public Header Block. The start of the first EVLR is at the file offset indicated by the “Start of first Extended Variable Length Record” in the Public Header Block. The Extended Variable Length Records must be accessed sequentially since the size of each variable length record is contained in the Extended Variable Length Record Header. Each Extended Variable Length Record Header is 60 bytes in length. Table 22: Extended Variable Length Record Header Item Format Size Required Reserved unsigned short 2 bytes User ID char[16] 16 bytes * Record ID unsigned short 2 bytes * American Society for Photogrammetry & Remote Sensing Page 20 of 27 LAS SPECIFICATION Version 1.4-R11
Item Format Size Required Record Length After Header unsigned long long 8 bytes * Description char[32] 32 bytes Note: As with the VRL, the Reserved field must be set to zero DEFINED VARIABLE LENGTH RECORDS: LAS 1.4 defines Variable Length Records (VLR) and Extended Variable Length Records (EVLR). A writer who wishes to maintain legacy compatibility must use only VLRs (except for internally stored waveform data). A writer who is not concerned with a legacy LAS reader having access to a VLR can elect to use an EVLR, even for predefined VLRs such as Coordinate Reference System (CRS) information. This ability is useful, for example, when a user wishes to update information normally contained within a VLR without the need of rewriting the point data. A new LASF_Spec “Superseded” (value 7) has been defined to allow a writer to indicate that a VLR should no longer be used. For example, if a user appends a new WKT EVLR to a file, the existing WKT VLR should have its LASF Spec number changed to Superseded to indicate that it is no longer in use. Coordinate Reference System Information The Coordinate Reference System (CRS) information for the point data is required for all data. The CRS information will be placed in Variable Length Records or Extended Variable Length Records (note that if the writer wishes to maintain legacy compatibility, then GeoTIFF in VLRs must be used). The CRS is represented by either GeoTIFF or Well Know Text as indicated by the WKT Global Encoding bit. Point Record Formats 0-5 can use either GeoTIFF or WKT (but not both simultaneously). Point Record Formats 6-10 must use WKT. Georeferencing Information using WKT For definition of WKT, we refer to Open Geospatial Consortium (OGC) specification “OpenGIS coordinate transformation service implementation specification” revision 1.00 released 12 January 2001, section 7 (coordinate transformation services spec). This specification may be found at www.opengeospatial.org/standards/ct. As there are a few dialects of WKT, please note that LAS is not using the “ESRI WKT” dialect, which does not include TOWGS84 and authority nodes. WKT georeferencing information can be specified in two optional variable length records, the OGC math transform WKT record and the OGC coordinate system WKT record, as follows. Note that the math transform WKT record is added for completeness, and a coordinate system WKT may or may not require a math transform WKT record (a parameterized math transform definition). American Society for Photogrammetry & Remote Sensing Page 21 of 27 LAS SPECIFICATION Version 1.4-R11
Page 1 and 2: LAS SPECIFICATION VERSION 1.4 - R11
Page 3 and 4: 2 Conformance The data types used i
Page 5 and 6: PUBLIC HEADER BLOCK: Table 3: Publi
Page 7 and 8: System Identifier: The version 1.0
Page 9 and 10: Table 6: Variable Length Record Hea
Page 11 and 12: Table 8: Classification Bit Field E
Page 13 and 14: Item Format Size Required Number of
Page 15 and 16: Z = Z0 + Z(t) where X, Y and Z are
Page 17 and 18: Scanner Channel: Scanner Channel is
Page 19: Item Format Size Required User Data
Page 23 and 24: struct sGeoKeys { unsigned short wK
Page 25 and 26: The bit mask in the options field s
Page 27: These records contain information t

LAS SPECIFICATION VERSION 1.4 – R11 - asprs

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